Bingowings said:
In that set up when Luke is saying "I can't do it alone" he is expressing doubts to Artoo over his ability to save everyone now that Leia has blown the plan rushing to save Han so Ben turns up to give him a bit of a pep talk but also to bring the story of his father's fall full circle.

The context changes the meaning of the line.

I don't think what you are talking about could every be done by any stretch of the imagination, but even if if could, wouldn't it just make more of a mess?

If you took Ben's telling Luke that Leia is his sister and placed it at the beginning of a movie, I guess you have to remove it from its rightful spot, to prevent redundancy. Since Ben has already confirmed that Vader is Luke's father and Leia is his sister, then Luke would have no reason to ask Yoda if Darth Vader was really his father, and Yoda's dying words "there is another Sky...walk...er" would be meaningless. In the attempt to fix some logical errors at the start of the film, you'd be bungling the entirety of the film and making some of the more meaningful moments less meaningful. Also, Threepio translates for Leia, so how could she already be capture by the time Luke decides to send the droids?